In the power conversion field, there is a trend toward the increase of converter operating frequencies, with the objective of reducing converter size, weight and cost. Increase of the switching frequency allows significant reductions in the size of converter transformers and reactive filter components.
Traditional pulse width modulation converters use transistor switching to terminate current flow at the end of each pulse. Transistor switching loss, which increases linearly with switching frequency, is a major impediment to the increase of switching frequency. Transformer nonidealities, such as leakage inductance and winding capacitance, also become more significant at higher frequencies and can further degrade converter efficiency.
Recent work on resonant switch converters has attempted to eliminate some or all of these sources of power loss and thereby attain good efficiency at higher operating frequencies. These classes of converters consist of linear resonant tank elements operated in conjunction with transistor power switches such that their transistors switch on and off at either zero voltage or zero current, and therefore exhibit greatly reduced switching loss. The resonant tank elements also often appear in series or parallel with transformer nonidealities, and therefore transformer leakage inductance and/or winding capacitance does not degrade operation of the converter. It may be possible to increase the converter operating frequency by one or two orders of magnitude using these techniques.
The U.S. Pat. No. 4,415,959 to Vinciarelli discloses a DC-to-DC converter with a switching device connected in series with a source of energy and an LC circuit effecting current pulses through the switch. A control circuit turns the switch on and off when the current is zero, thus eliminating switching losses.
The paper by Liu and Lee, "Resonant Switches--Topologies and Characteristics," IEEE Power Electronics Specialists Conference, 1985 Record, pp. 106-116, discusses half-wave and full-wave configurations of resonant switches and their application to converter circuits of various kinds including buck, boost, and buck/boost resonant switch converters.
The paper by Ngo, "Generalization of Resonant Switches and Quasi-Resonant DC-DC Converters," IEEE Electronics Specialists Conference, 1987, pp. 395-403, further discusses and analyzes the resonant switches as applied to converters.
It is generally recognized that pulse-width-modulated (PWM) converters of various kinds can be adapted to resonant switch operation to achieve zero current switching and therefore realize no switching losses. On the other hand, previously proposed resonant switch converters have the disadvantage that, for the same output current and power, resonant quasi-sinusoidal waveforms exhibit higher peak values than do the rectangular waveforms of conventional pulse-width-modulated converters. Consequently, resonant switch converters exhibit higher conduction losses in the transistor and these increased conduction losses can partially or wholly negate the gains made by the elimination of switching losses. Even though power FET's have desirable characteristics for switching in power conversion devices, they do have significant resistance. Thus it is important to minimize the peak current to minimize the losses.